These mechanical devices include two shafts, arranged in coaxial relation with each other as splined shafts with various shapes, one connected to gripping means, the other kept pushed against the first one, against the action of elastic means and driven into rotation by a motor.
During tightening of a cap, gripped by said gripping means, onto a corresponding bottle, the motor is operated and the just defined group moves with a rotation-translation motion, until the cap is completely screwed.
The transmission of the torque between the two shafts, and thus of the rotary motion, is assured by the friction coupling, which occurs between the corresponding splined profiles and which depends not only on the profiles shapes and contact surface, but also on the material, from which they are made and on the mutual pressure characterizing their contact.
In conditions of normal operation of the motor, that is when the cap is screwed on the corresponding threading in a regular way, a prefixed torque is transmitted to the driven shaft, so as to cause, in this order, a rotation of the latter by a prefixed angular pitch and a determined tightening of the cap.
When the screwing is jeopardized, for example, by mechanical inconsistencies of the two kinematic elements in question, the motor delivers a higher torque with respect to the prefixed one, putting at risk the coupling and the integrity of the gripping means.
The above described friction coupling imposes an upper limit to the maximum transmittable torque, which will be calculated in such a way, as to keep safe the gripping means, the cap and the threading, on which the cap is to be screwed: consequently, if the resisting torque value goes beyond the above mentioned limit value, the two connecting shafts slip one with respect to the other.
However, the elastic means aimed at transmitting the motion and at defining the “maximum transmittable torque” of the above mentioned mechanical devices, must be adjusted periodically because the mechanical features of these latter decay gradually with time.
Otherwise, as it has already been said, friction systems are used, which generally have also a friction coupling between the gripping means and the cap to be screwed onto the open head of a corresponding bottle; the added friction device comes into operation only in anomalous conditions, when the screwing step becomes irregular and the applied torque reaches or exceeds the allowed maximum value, this way causing the slipping of the gripping means on the cap.
Also in this case, the maximum allowed torque must be valued on the ground of the coupling characterizing the gripping means and the cap gripped by the latter.
Therefore, if the type and/or dimensions are to be changed, it is necessary to verify whether the new maximum torque value damages, if reached, the physical integrity of the cap and of the threading of the bottle, onto which the cap is screwed.
In some cases, the above described mechanical and friction systems can be provided with torque limiting devices, associated to the power means, so as to intervene during the tightening of the cap on the corresponding bottle, when the preset torque value is reached; in this way, the slipping of the connecting shafts, in one case, and the slipping of the friction means on the cap, in the other, occur only in isolated and accidental cases, when the limiting devices do not operate properly.